On Hot-rolled and Cold-rolled Malleable Iron, 151 



that the solid crust of the earth cannot be less than 800 miles thick. 

 The author indeed believes it to be extremely improbable that any 

 crust thinner than 2000 or 2500 miles could maintain its figure with 

 sufficient rigidity against the tide-generating forces of the sun and 

 moon, to allow the phenomena of the ocean tides and of precession 

 and nutation to be as they are. 



"On the Difference in the Properties of Hot-rolled and Cold- 

 rolled Malleable Iron, as regards the power of receiving and retaining 

 Induced Magnetism of Subpermanent Character. 5 ' By George Bid- 

 dell Airy, Esq., F.R.S., Astronomer Royal. 



The author states that he had been desirous of examining whether 

 differences in the degree of change of subpermanent magnetism, such 

 as are exhibited by different iron ships, might not depend on the 

 temperature at which the iron is rolled in the last process of its 

 manufacture. By the good offices of Mr. Fairbairn he had received 

 gratuitously from Richard Smith, Esq., Superintendent of Lord 

 Dudley's Iron Works at the Round Oak Works near Dudley, twenty- 

 four plates of iron, each 16 inches long, 4 inches broad, and \ inch 

 thick ; twelve of which, after having been manufactured with the 

 others in the usual way, had been passed through rollers when quite 

 cold. Each set of twelve was divided into two parcels of six each, 

 one parcel being cut with the length of the bars in the length of 

 extension of the fibres of the iron, the other being cut with the 

 length of the bars transverse to the length of extension. 



For experimenting on these, a large wooden frame was prepared, 

 capable of receiving the 24 bars at once, either on a plane transverse 

 to the direction of dip at Greenwich, or on a plane including the 

 direction of dip. In some experiments, these planes were covered 

 with flag-stones, and the bars were laid upon the flag-stones ; in 

 others, the bars were laid immediately upon the wood. While there 

 lying, they were struck with iron or wooden hammers of different 

 sizes. The bars of the different classes were systematically inter- 

 mingled, in such a way that no tendency of the arm to give blows of 

 a different force or kind ifl special parts of the series could produce 

 a class-error in the result. For examination of the amount of polar 

 magnetism in each bar, it was placed at a definite distance (5 inches) 

 below a prismatic compass, which was used to observe the apparent 

 azimuth of a fixed mark ; the bar was then reversed in length, and 

 the observation was repeated in that state. 



The number of»-experiments was 21. They were varied by differ- 

 ence in the succession of positions of the bars, difference of time 

 allowed for rest, difference in the violence of the blows, &c. 



The principal results appear to be the following : — 



1 . The greatest amount of magnetism which a bar can receive, 

 appears to be such as will produce (on the average of bars) a com- 

 pass-deviation of about 11°, the bar being 5 inches below the com- 

 pass. It was indifferent whether the bars rested on stone or on 

 wood, or whether they were struck with iron or with wood, the bars 

 lying on the dip plane while struck. 



